The present invention relates to video systems, and more particularly, to a method and apparatus for greatly enhancing the quality of video displays generated from conventional video signals.
Video images are generated on a CRT through the motion of its electron beam. This motion consists of scanning from left to right with a quick return to the left again, and from the top downward in subsequent lines. At the bottom of the image, the electron beam returns to the top left corner, and the process is repeated. Typically, successive fields are interlaced, i.e. the lines of the second scanning field fall between the lines of the first scanning field. This technique provides two picture images or fields for each frame and thereby greatly reduces flicker.
The number of scanning lines in the image is related to the resolving capability of the human eye and the viewing distance. It has been determined that if a pair of parallel lines is viewed at such a distance that the angle subtended by them at the eye is less than two minutes of arc, the eye sees them as one line. This fact is used to select the number of lines for a television system.
The NTSC standard followed in the United States provides 525 lines displayed at 30 frames per second, each frame consisting of two interlaced fields of 262.5 lines, the fields being displayed at 60 times a second. Each image is created from separate red, green and blue (RGB) images. An empirically derived matrix is used to convert these to luminance and chroma signals. Each chroma signal is delibertely bandwidth limited and modulated in quadrature on a 3.58 Megahertz subcarrier and superimposed on the corresponding luminance signal. This rather complex method of transmitting color video was defined in order to introduce color within the existing channel bandwidth in a manner which was compatible with the previously established monochrome standards.
Most video as seen in the home or professional environment does not exhibit the full 525 line resolution that is possible with the NTSC signal. A "good" home set will actually display about 300 lines. Only production broadcasters and other special purpose practitioners use "studio" quality monitors which can display the full 525 lines resolution. The projection video sets commonly sold for home use typically have even lower resolution, for example 220-240 lines.
It is necessary to distinguish between resolution and subjective image quality. As previously indicated, resolution depends upon the ability to distinguish between two lines placed close to each other. Better quality images as defined by observers, usually, but not always, will have higher resolution. The ability to increase the resolution of a given image using any kind of image processing is greatly limited, and often depends upon a prior knowledge of the scene content. However, great improvements in observer reported image quality can often be gained in other ways. In general, observers prefer images that are bigger and brighter. They will often rate images that have artificially enhanced edges of the objects in the scenes higher than the same images when accurately reproduced. This is well-known in the video art. Many of the best video cameras do not capture accurate reproductions of scenes, but rather introduce a halo around bright objects. This results in a more pleasing picture without increasing the resolution.
In many ways, video resolution correlates with the bandwidth needed to transmit the signal. Modern techniques can be employed to reduce the bandwidth needed for a given resolution. However, throughout the world the format and content of video signals are rigidly established. In part this is because transmission bandwidths are allocated and regulated by government agencies. Accordingly, if an improvement in the quality of video displays is to be achieved, it must be done within the constraints of the information available from conventional video signals. For example, it is impractical to suggest that resolution be improved by increasing the number of scanning lines per frame in the original video signal without government support.
Besides improving the resolution of images in a video display, it would also be desirable to improve the signal to noise ratio, sharpen the edges of objects in the displayed images, and enable color value manipulation.
Heretofore digital techniques have been utilized to perform image processing tasks to improve video displays. For example, still images transmitted back to earth from space probes have been processed on large computers to greatly improve signal to noise ratio and enhance certain aspects of the image. However, moving objects cannot be displayed in real time through this process as the computations required are extensive and time consuming.
A set of specialized circuit boards sold by Imaging Technology, Inc. of Woburn, Mass. under the designation IP-512 perform video signal processing to improve signal to noise ratio. This system is described in an article entitled "Video Signal Processing Improves SNR" by Robert Wang published in Electronic Imaging Magazine, March, 1983, pages 36-41. Because of speed limitations in the IP-512 system, the frame average and summation techniques disclosed in that article cannot currently be performed in real time on images of a moving object. The image compression and expansion techniques described in that article do not improve image resolution.